1. Field of the Invention
The present invention relates to a tool to be attached to a spindle of a machine tool for machining a workpiece.
2. Description of the Related Art
In for, example, a machining center or other machine tool provided with a spindle, the maximum rotational speed of the spindle (per unit time) is determined by the structure of a main bearing rotatably supporting the spindle and a lubrication system. For this reason, when rotation of a tool at a higher rotational speed than the maximum rotational speed of the spindle is desired, an accelerating apparatus is used.
As accelerating apparatus which holds the tool and is able to be attached to the spindle and which can increase the rotary force of the spindle by a gear mechanism such as epicyclic gearing to increase the rotational speed of the tool is known.
For example, in a machining center, when it is desired to increase the rotational speed of the tool to higher than the maximum speed of the spindle temporarily, such an accelerating apparatus is attached to the spindle in the same way as an ordinary tool to enable the tool to be rotated at a higher rotational speed.
However, when raising the rotational speed of the tool to a higher speed than the spindle by the above gear mechanism such as at a super high rotational speed such as tens of thousands to hundreds of thousands of revolutions per minute, the accelerating apparatus increasingly generates heat so the machining tolerance of a workpiece can be influenced by the heat. Further, at the above super high rotational speed, the noise from the accelerating apparatus can also increase. Furthermore, a highly reliable precision structure able to withstand the above super high rotational speed is required for the accelerating apparatus. Therefore, there is the disadvantage that the manufacturing cost becomes relatively high.
Further, in a case of an accelerating apparatus with a gear mechanism, it is necessary to lubricate the gear or bearing and arrange a supply passage and a discharge passage for the lubricating oil in the accelerating apparatus, so there is the disadvantage that the apparatus becomes larger and it is difficult to automatically change the tool by an automatic tool changer.
Further, as another accelerating method, sometimes the method is adopted of using a high frequency motor for the motor driving the tool and supplying drive current to this high frequency motor from a specially provided control apparatus so as to rotate the tool at a high speed. With this method, however, since there is a cable for supplying electric power from the outside, there are the disadvantages that it is difficult to automatically change tools like with an ordinary tool and the cost of the facilities is relatively high.